1. Field of the Invention
The present invention relates to a method for measuring each motor parameter required for vector control operations of an induction motor, and to a control apparatus for an induction motor comprising a measuring system.
2. Description of the Prior Art
Vector control systems having both excellent response and precision have been put to practical use for variable speed control of induction motors. In this vector control, an input current to an induction motor is separated into an excitation current for generating a magnetic flux and a torque current for generating a torque. As the motor torque is proportional to the torque current when the excitation current is constant, the excitation current and the torque current are controlled according to a torque command value and the rotational speed of the induction motor.
FIG. 8 shows a block diagram of a conventional control circuit of an induction motor using vector control. In FIG. 8, when a speed command .omega..sub.m * and a motor speed .omega..sub.m, output from a speed estimator 9 of an induction motor 8, are input to a speed controller 1, it outputs a torque current command I.sub.1q * according to a difference between the two inputs. This block diagram relates to speed control. However, for torque control, the speed controller is replaced by a torque current command means for transforming a torque command to a torque current command I.sub.1q * to be output.
The torque current command I.sub.1q * and an excitation current command I.sub.1d * are input to a slip frequency operator 2, which outputs a slip angular frequency .omega..sub.s. The excitation current command has a predetermined value determined by the induction motor.
The sum of the slip angular frequency .omega..sub.s and the detected speed .omega..sub.m, that is, a rotating magnetic field velocity .omega..sub.1, is input to an integrator 3, which integrates it to output a phase .theta.. The torque current command I.sub.1.sub.q * the excitation current command I.sub.1d *, and the phase .theta. are input to a rotary/stationary coordinate transformer 4. The output of the rotary/stationary coordinate transformer 4 is transformed to a three-phase current command in a two-phase/three-phase transformer 5 to be provided to a current controller 6.
The current controller 6 produces a PWM (Pulse Width Modulation) signal from the three-phase current command to be output to a driver 7. The driver 7 generates a drive voltage from the input PWM signal to be provided to the induction motor 8 for drive. The input current to the induction motor is fed back to the current controller 6 to be controlled to follow the three-phase current command.
In the speed estimator 9, a motor speed is estimated by using a detected or commanded voltage of the induction motor, a detected motor current, and set motor parameters.
In FIG. 8, the rotational speed of the induction motor is found by the speed estimator 9. However, it may be detected using a speed sensor.
Motor parameters are used for the operation of a slip frequency in the slip frequency operator 2. Also, motor parameters are used for an estimation operation in the speed estimator 9. Therefore, motor parameters should be obtained by some method before motor running.
Conventionally, stator winding resistance measurement, a no-load test, and a rotor locked test are known as methods for obtaining the motor parameters of an induction motor. Also, a method requiring the no-load rotation of an induction motor as described in Laid-open Japanese Patent Application No. (Tokkai hei) 7-75399, and a method requiring a rotor locked test as described in Laid-open Japanese Patent Application No. (Tokkai hei) 6-153568 are proposed.
However, it is difficult to create a no-load state or a rotor locked state in the measurement of a motor already installed in facilities. As a measurement method with a motor at rest, there is a method based on a model accommodation system as described in Laid-open Japanese Patent Application No. (Tokkai hei) 2-304380. However, this method has a disadvantage in that operations are complicated.